Bilayer design: The secret to a good lift-off process

I often come across researchers who struggle with a lift-off process: They usually write decent patterns with electron beam lithography, do a good metal evaporation, but when it comes to lifting off the polymer, everything gets ripped apart including their metallic structures.

Most of the time, it's because the evaporated metal coats the polymer sidewalls. To do a better lift-off, it's best using a bilayer design where you have a higher molecular weight polymer on top of a lower molecular weight polymer, as shown in the figure below.



When you expose the bilayer with ebeam, an undercut is formed. Due to the undercut, you avoid coating the sidewall of the resist when you evaporate metal. Lift-off then becomes a breeze!